Time-Resolved Assembly of Chiral Uranyl Peroxo Cage Clusters Containing Belts of Polyhedra

Two chiral cage clusters built from uranyl polyhedra
and (HPO3)2– groups have been synthesized
in pure
yield and characterized structurally and spectroscopically in the
solid state and aqueous solution. Synthesis reactions under ambient
conditions in mildly acidic aqueous solutions gave clusters U22PO3 and U28PO3 that contain
belts of four uranyl peroxide pentagonal and hexagonal bipyramids,
in contrast to earlier reported uranyl peroxide cage clusters that
are built from four-, five-, and six-membered rings of uranyl hexagonal
bipyramids. U22PO3 and U28PO3 are also the first chiral uranyl-based cage clusters, the
first that contain uranyl pentagonal bipyramids that contain no peroxide
ligands, and the first that incorporate (HPO3)2– bridges between uranyl ions. They are built from 22 uranyl polyhedra
and 20 (HPO3)2– groups, or 28 uranyl
polyhedra and 24 (HPO3)2– groups, with
the outer and inner surfaces of the cages passivated by the O atoms
of uranyl ions. Small-angle X-ray scattering (SAXS) profiles demonstrated
that U22PO3 clusters formed in solution within 1 h after mixing of reactants,
and remained in solution for 2 weeks prior to crystallization. Time-resolved
electrospray ionization mass spectrometry and SAXS demonstrated that U28PO3 clusters formed
in solution within 1 h of mixing the reactants, and remained in solution
1 month before crystallization. Crystallization of U22PO3 and U28PO3 is accelerated by addition
of KNO3. Clusters of U22PO3 with and without encapsulated cations exhibit
markedly different aqueous solubility, reflecting the importance of
cluster surface charge in fostering linkages through counterions to
form a stable solid.